Friction article



Aug. 16, 1960 A. w. ALLEN ETAL 2,948,955

FRICTION ARTICLE Filed Oct. 8, 195'? 2 Sheets-Sheet l R l 1N 1\\ i w u m.A a Ra www. 4 A ff 1 Z d E W O M Q Q0 ,d w M @A WW 3 W -l o M, M m o wom.. A

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FRIcTroN ARTICLE Alfred W. Allen, Urbana, I1l.,'andRobert H. Herron,

South Bend, Ind.,vassignors to Bendix Aviation Corporation, SouthrBend,Ind., a corporation of Delaware Filed Oct. 8, 1957, Ser. No. 688,917

11 Claims. (Cl. 29-1825) This invention relates to a friction articleand more particularly to an improved friction material composition andto the combination of the composition with a suitable retainer, for usein high kinetic energy braking such as aircraft braking.

The present invention constitutes an improvement over the brakingcomposition disclosed in the' copending Pocock and Stedman applicationSerial No. 257,162, iiled November 19, 1,951, now abandoned, acontinuation of this application being application Serial No. 545,637,led November 8, 1955, and nowy U.S. Patent No. 2,784,105, issuedMarch 5,1957, the method for making the friction article being illustrated inPocock application Serial No. 257,292, filed November 20, 1951, nowabandoned. Application No. 600,808 forms a continuation-impart 'ofapplication No. 257,292; this continuation-impart `wasiiled July 30,1956, and superseded applicationNo. v257,- 292. The present applicationis a continuation-impart of application Serial No. 355,586 filed May'18, 1953, now abandoned. e c

The service requirements of aircraft brakes have exceeded the capacityof conventional organic linings to such an extent that brakingcompositions of the organic type will eventually be displaced to a largeextent by ceramic-metallic linings of the `type disclosedin'applicationSerial No. 257,162, reference to which is* mad'e'for a statement offurther advantages of the ceramicrnet'allic lining. In general, it canbe stated that brake lining compositions having a high concentration ofceramic friction material have proved to combine the advantages .of moreeffective braking and longer service life. f

'It hasl been found by experimentation, and it willbe later illustratedgraphically, that while increasing the amount of the ceramic primefriction material sucli as calcined kyanite` will improve theeifectiveness'rand decrease wear of a ceramic-metallic lining, the highconcentration of ceramic tends to accentuate the fslip-stic frictionproperty of lthe brake, that is, the tendency o f the brake toalternately graby and release near the end'ofthe braking cycle, acondition commonly referred to as chatter. When chatter of the brake issought to be eliminated the remedy must, in some Way, be related to thecause of the condition and hence the nature 'of what produces and whatis yassociated with chatter of the brake has been investigated. Chatteris related to the difference in static and kinetic coefficients offriction of the engaging surfaces of the brake. Y between the static andkinetic coeicientsof friction Vbecomes great or when they become very'greatly*disproportionate then this is an indication that brakechatteringWhen 'the difference niedS/efs Pate Patented Ang, 1&6, 1,960

of the braking cycle, owing to an increase in the braking Yeffectiveness with decreasing velocity.

vIt has been found by experience that this torque buildup condition isassociated with a glaze formati-on on the surface of the frictionarticle which consists predominately of voxidation productsY of highvalence iron. Y which are formed under the inuence of heat absorbed Theconsistent association of' during the ybraking cycle. c the high torquebuild-up which causes chatter and the formation of high melting glaze onthe surface of the lining leads to the reasonable inference that a highmelting point glaze is one of the primary causes of chatten As anillustration of braking conditions encountered,

the surface `of the friction article may be elevated to' temperaturesabove approximately a red heat and believed to be about 2500 F. las theaircraft is braked during a landing operation, and it is under theiniluence o'f such high temperatures that objectionable high meltingpoint glaze is formed with oxidation products of iron. It is therefore'an object of the present invention to eliminate the Yexcessive increasein torque delivery by the brake at the Aendof the `stop which tends tosupport chatter. Y n Y h h Y 'Another' object of the invention is toobtain a brake lining which will tolerate ahigh concentration offriction-producing ceramic vin order to achieve amore effective brakewith a longer service life, but will yet avoid .the eitect of brakechatter heretofore associated with higher concentrations offriction-producing ceramic.' l

Another object of the invention is to produce a friction article whichis self-compensating for 'effects of oxidation induced by high heatsabsorbed by the brake from kinetic energy of the aircraft.' A

Another` object of the invention is to provideY a brake lining whichYwill not coat engaging surfaces during a brake application. l Y Theabove and other objects and features ofthe invention will appear morefully hereinafter from a consideration of the following description,taken in connection with the accompanying drawings illustrating thefriction article, the test equipment, and performance of a plurality fof braking compositions which are described by Way of example.

In the drawings: Y

' specimens;

Figures 2 and 3 are recordings illustrating 4the torquespeedcharacteristics obtained from dynamometer testings of aircraft brakesutilizing 4ceramic-metallic linings;

Figure 4 is a photomicrograph of a sectionV of the lining after a seriesof high speed braking applications;

Figure 5 is a cross section of the friction article.

The generally desirable characteristics of a ceramicmetallic brakeinclude its ability to withstand the high temperatures encounteredduring high speed stops which are characteristic of aircraft braking.Surfacev temperatures at least as high'as 2500 F. are not uncommon inthis eld,and one of the effects of high heat absorbed by the brake ispartial oxidation of ferrous retaining member l 10, friction composition12 being compacted and sinteredl therein to form the complete frictionarticle 14. This surface temperature of at least 25 00 F. is establishedby identifying fayalite formation upon analysis of the article after aseriesof braking tests. Observation indicates that a high melting pointvg la2fe composed predominatelyrof oxidation products of high valenceiron cover the surface of the lining. Experimentation indicates thatthis type of glaze is a cause of the excessive torque build-upsupporting slip-stick characteristics of the brake.

The preferred friction composition 12 consists broadly of apredominately copper metal matrix, a prime frictionproducing heattreated ceramic consisting predominately of mullite, a ca rbonaceouslubricant such as graphite, forV Sonie applications a matrix-insolublesecondary frictionproducing agent such as iron, and varying amounts ofan antioxidant in the form of molybdenum or tungsten. The purpose of themolybdenum is to control the valence state of the iron in its oxidephase to maintain a low melting point glaze Broadly speaking, threefactors account for the irnproved results obtained with the preferredfriction article 14; the first of which involves providing a ferrousretaining member 10 to permit formulating the frictionproducingcomposition 12 more with a view to such properties as wear andeffectiveness and lessA to the strength ofthe compact, this functionbeing supplied by the ferrous retainer l0. The second factor has to dowith increasing the prime friction-producing ceramic in the cornpositionto multiply the number of stops which the brake can deliver and to givegreater braking effectiveness, while at the same time minimizing theeffect of high temperatures on braking qualities. The third factorrelates to the use of an antioxidant such as molybdenum or tungsten formaking the lining self-compensatory as to effects of oxidation whichaccompany braking conditions thereby preventing brake chatten acondition heretofore associated with higher concentrations of ceramicfriction material.

The function of each of the constituents of the liningmatrix, frictionproducing material, lubricant, and antioxidant is best understood ifconsidered separately and in relation bothV to eliminating chatter andto the other requirements of the brake, such requirements being amongothers; wear, effectiveness, and mechanical strength. It is to beunderstood, however, that these constituents are interrelated andnecessarily affect to some degree each others function.

. The matrix both in composition and amount is lgenerally related to theheat conductivity of the brake, its mechanical strength effectiveness,and also, to some extent, to the tendency of the brake to supportchatten To establish the limits defining the range of matrix materialwhich would give improved chatter characteristics, lining VspecimensVwere made up with graduations in amount of matrix material varying from20% to 90% of the lining. These specimens were then tested on a buttontesting machine 16 of the type illustrated in Figure l, the operation ofwhich will be explained later in this description. A series of brakingapplications of the specimens were then made and by calipering thethickness of the specimens after these applications an average value ofthe wear per surface per stop could be calculated. Braking effectivenessof the specimens was also determined within the range of 20%-90% matrix.As a result of these wear and effectiveness tests it was determined thatthe matrix could be varied between the approximate limits of 35 %65optimum relationship between metals and non-metals was achieved. Thatis, there was sufficient matrix to bond the friction-producing ceramicand yet enough ceramic to produce sufficient braking effectiveness.

Other considerations were taken into vaccount in determining the optimumrange of matrix material. These additional considerations involve theVthermal conductivity f the brake lining and also the tendency of thebrake lining to support chatten These additional factors will next beconsidered in relation to establishing the Opti,-`

mum range of matrix material. Since the glaze formed on the surfaceofthe lining is the result of high temperatures 1n the brake, the matrixis related to the problem ciated with such brake lining compositions.Within this range an of glazingf and hence torque build-up, by itseffect on heat conductivity of the lining and therefore operatingtemperatures. With a small amount of matrix material the thermalconductivity of the lining is low, resulting in surface temperatureswhich are sufficiently elevated to cause a controlled melting of theglaze, thereby tending to prevent torque build-up. With the lowerpercenttages of matrix the strength of the friction material isdiminished and the rate of wear of the lining is greater; therefore,these considerations dictate the lower limits of matrix content. Forpresent braking purposes this lower limit is established atapproximately 35% by weight of matrix.

As the matrix is increased above approximately 65% the brake compositiontends to have lower effectiveness and greater tendency to supportchatten Above this upper limit of matrix content the effectiveness isreduced because of the smaller amount of friction-producing ccramicmaterial. Also, the higher amounts of matrix reduce the surfacetemperature of the lining by reason of greater heat conductivity of thefriction article. This substantial reduction of surface temperature isthought to be a factor contributing to chatter of the brake,l sincethere is less melting of the glaze Another important factor to beconsidered in relation to chatter is the effect of the matrix as alubricant. With lower amounts of ceramic there is sufficient matrixexposed to the mating surface of the rotor so that the metallic matrixcontributes to lubrication. When the ceramic content is increased,however, the lubrication at the surface of the lining is not influencedto as great a degree by the matrix and hence must be controlled by meansother than the matrix, in a manner which will be explained more fullylater in this description.

VThe preferred prime friction producting material used in the lining isa heat treated ceramic, predominately mullite (3Al2O3.2SiO2). Mullitemay be derived from following the two methods:

(A) Heat treating stoichiometric quantities of alumina and silica in aprocess to produce mullite by crystallization from a melt.

(B) Sintering stoichiometric proportions of alumina and silica toproduce mullite.

The alumina and silica in either method may be derived fromnaturally-occurring materials such as minerals of the sillimanite group,topaz or argillaceous raw materials having as their primary constituentsminerals of the kaolinite or aluminous groups (bauxi-tic, diasporitic orgibbsitic). Varying amounts of silica (Si02) may be added to increaseeffectiveness of the lining and decrease Wear without contributing tobrake chatten As a general proposition, within limits the greater theconcentration of prime friction producing material, the more effectivethe brake and the greater the service life, but there exists theaccompanyingv disadvantage of greaterV torque build-up-at the end of thestop. lt is one of the important aims of the present invention toprovide a brake lining composition having high concentrations ofceramic, with the accompanying advantages, but without the excessivetorque build-up characteristics often asso- By following the teaching ofthe present invention it is possible to obtain improved brakingcompositions with as high as 43% of ceramicV material. A second ceramicsuch as SiO may be used to supplement or augment the friction effectproduced bythe prime friction producing material. The total amount ofceramic used is limited by its tendency to cause chatter and decreasethe strength of the lining. The secondary friction producing' ceramiccan be entirely' eliminated for some braking usages.

In development work prior to the present invention, lining specimenswere fabricated varying the amount of thisV material-in` graduations of5% within the limits of 10%Y to about 50% Vby weight of the compositionin order to determine the optimum range of ceramic material.

AS' a`result` o'f these tests it ws^found that the'amolint of torquebuild-up increased slightly as the ceramic con tent was increased fromto 20% by weight of the lining; further additions of ceramic tended todecrease the torque build-up until the lining was about 30% ce- 5 ramicmaterial. Further additions Yof ceramic had the efectrof increasingtorqueWbuild-up Vwhich eventuallyV caused inoperability of theliningtwhen the ceramic content reached about 50% by weight oftherlining.Y Y Y The effect of varying the ceramic content on other 10operating characteristics of the lining was simultaneously` determined.The effect of ceramic content on the wear patternv of the 'lining is asAfollows" it' waswfound that'farw low ceramic content lining had highwear; increasing the ceramichad th'e' effect of decreasing wearp'rsurfac'e per 15 stop until the ceramic content was about 43% of thelining. Beyond this concentration of ceramic the rate of Wear increasedsharply, indicating that there was insufcient matrix to form adequatebonding with the fric-v tion producing material.

The elfectiveness of the lining is also related to the i' ceramiccontent. It appears to be generally true that the higher concentrationsof ceramicV tendfto produce more .i effective linings; however, VthereisA evidence'tending to' indicate thatthis relationship is not directlyproportional,

' is composed largely of oxides of iron in its highest and that optimumresults may be obtained in the range of 20% to 35% (total Yceramiccontent).

Taking into account each of the'factors of wear, effectiveness and'torque build-up, Vthe results of these testsin-fY Y dicated thatanoperative brake lining can be provided by* compositions which have aceramic content varying fromVA about 15% to 43% by weight of the lining.The tests performed to 'establish theY maximum4 operative range weremade on the button testing machine described here-m in. For conclusiveresults dynamometer testsareneces-35 sary. While numerousdynamometer'tests have been car- 11V ried out.V on compositions of Vthetypedisclosedrherein it has been impossible, for reasons of economyandtime, j to performgall the dynamometer tests toestablish the entirerange of ceramic material. dynamometer tests (and in some instances,flight tests) Y have been made to prove the satisfactory operationofallMv Y 7 the compositions listed in Table I below. rBased on thedynamometer and ight test results, it is believed that rthe optimumrange of Vtotal ceramic content is approximately 45 21%, 19.31%- L i.jg., it; A lubricating material suchY as grapln'te orrequivalent"Wcarbonaceous material may beadded in varying amountsv Y to furtherreduce the tendency of the brake to chatten The required amount ofgraphite is dependenton the other constituents of the lining and theextent to which they already contribute to its lubricating qualities.The amount of the graphite or .other lubricating'material'is governed bydesign requirements. Amounts which have proved satisfactory in actual-dynamometer tested com-y positions showing more satisfactory torquebuild-up characteristics have varied from 8% to as little as 1.1%', andconceivably this material could be deleted entirely if properlycompensated for by increasing the lubricating effect of one or more ofthe other constituents of the lining.

A metallic lubricant such as lead may be included in the composition forsome usages. The lead addition has varied vfrom 2.0%-3.7%. Plasticdeformation of the lead is believed to occur as well as some meltingduring the braking cycle in such a manner that this ingredient serves todecrease the coefficient of friction of the lining and acts as alubricant.

i For some applications, we have found that adding amounts of arefractory, matrix-insoluble metal such as iron helps to increase theeffectiveness of the brake and prolong the wear life of the liningwithout appreciably contributing to brake chatten The v'amount of ironor equivalent e material is v aried depending upon the e brakingcharacteristics and wear pattern desired.` While However, sufficientVVV40 this ingredient is not absolutely essentialto obtain 'the improvedbrake lining characteristics of the, present invention, yet it is usefulfor contributing to a greater effectiveness without appreciablycontributing to brake chatter. Amounts have been used in the range of 5%to 15 The actual amount used is not critical but may be varied to suitrequirements. It has been found that the iron also serves as a scouringagent which acts to remove any braking composition transferred to theengaging sur- -faces on the relatively movable part of the brake.

According to the principles of the present invention,

-a significant improvement in'brake composition is provided by an`antioxidant such as molybdenum. The function of this material is toserve as an oxygen-getter, which function it accomplishes by reason ofits affinity for free oxygen. As explained previously, there is evidencetending to indicate that the torque build-up which occurs `at the end ofthe braking cycle is caused by a film of high melting point glaze whichis formed over 0 the surface of the friction article and whichexperimentation indicates as including oxides of iron originating bothfrom the sides of the ferrous retaining cup and the mating surface ofthe rotor which are oxidized during the -cours'e of braking.

Chemical, microscopic, and X-ray analyses indicate that theobjectionable type of glaze valence forms. The molybdenum shows definiteindications of accomplishing reduction (and elimination) of YYchatter byinhibiting the formation of higher melting point'glaze over the surfaceof the friction article. Since'molybdenum has a greater affinity foroxygen than iron, it -acts to prevent the existence of iron in itshighest e valencerform thereby maintaining a lower melting point glazeAs supporting evidence, Fayalite was identified in the glaze FayaliteV(ZFeOSOZ) is stable only in an'environment of the lowest valence statesof iron.

Conceivably, other metals having an affinity for oxygen A lwould operatein the same manner as molybdenum and accomplish the same desirableresult. Tungsten has been tested and has shown generally the samefunction as molybdenum. Molybdenum has been used and proved 1'satisfactory.

j To determine the optimum amount of molybdenum,

lining specimens were formulated with graduated amounts ofmolybdenum inthe range of 1.0% to 12.5% by weight of the lining. These specimens werethen dynamometer testedto obtain torque-speed recordings from whichchat- 'ter characteristics could be determined.

The testing results indicate that amounts of molybdenum may be used inthe range of from about 1% to proportionate change in effect on brakingchatten Tests have also revealed that the particle size of themolybdenum has an effect upon its function in'reducing brake chattenParticle sizes of molybdenum in the range of -200 to `300 mesh haveproved satisfactory. From a consideration of testing results, it can bestated that the eect of molybdenum depends upon the heat conductivity ofthe lining and the measure in which it affects the operatingtemperatures. In other words, the function of molybdenum as anantioxidant becomes most lapparent when the ratio of metals to nonmetals is low, and surface temperatures are sufficiently elevated tocause the glaze to soften and/or melt. Y e

A useful generalization that may be applied is that the higher energyabsorbing usages require higher ceramic content in the lining, therebynecessitating the use of molybdenum. It is, therefore, apparent that the'amount of'antioxidant actually used in the lining depends on the otherconstituents' and the service needs.

Finely-divided molybdenum, g preferably in particle sizes of 200 to 300mesh, is mixed with the'nelydivided matrix metals and ceramic material,and a selected quantityk of the material is measured out and compactedin a suitable die by the use of pressures in the neighborhood of 40,000toY 100,000 lbsfper square Examples of lining formulations which have`shown reduced torque build-up based on either dynamometer test(simulating actual braking conditions) or actual aircraft braking underservice conditions, are yas follows:

Table l brass chips iron l Y 15.0 15.0 T6631 57.3 66.3 55.3 59.7 A59.956.7 62.3 64.6 64.1 61.2

Friction Material: l

631611166 kyanite--. 26.1 26.1 26.1 24.9 19.9 25.6 24.6 24.6 24.6 22.9511165 4.3 4.3 4.3 4.6 4.6 4.3 4.4 4.6 4.5 4.3

Lubricant:

graphite 1. 2 1. 2 1. 2 1.1 6. 1 1.1 1..1 1.1 1. 1 1. 6 ard mal l. 6 1.5lead 2.0 3.7 2.8

Antioxidant: i y i i m61ybdenum 16.6 7.5 12.5 9.6 9.5 9.3 4.6 2.3 4.79.1

Total 16.6 7.5 12.5 9.5 9.5 9.3 Y4.6 2.3 4.7i 9.1

Matrix:

copper 43.6 13.6 45.6 45.6 45.6 42.9 46.2 41.7 46.2 46.2 zine 16.6 16.616.6 9.5 5.5 16.6 5.5 5.5 tin-- 7. 4 3. 3 3. 3 3. 3 3. 3 nickel 6. 6 7.1 titanium 3.3 3.6 brass rhins 28. 6 ironl 5.6 3.6 16.6 7.6 3.6 3.6 7.631.3 Total y66.9 57.9 66.6 63.6 65.6 66.6 63.6 63.6 62.6l 63.3

Friction Material:

. 651611156 kyanite--. 24.2 26.1 26.6 26.6 26.6 19.6 26.6 26.6 17.5 22.6511163 4.5 4.3 7.6 4.6 2.6 3.3 4.6 4.6 3.5 4.4

Lubricant:

graphite 1.1 1. 2 3. 6 3. o 3. 6 7. 6 3. 6 3.6 7. 6 3. 3 hard rnai laadAntioxidant: l I l I I molybdenum 9. 3 16. 6 5.6 5. 6 5. 6 9. 5 5. 6 5.6 16. 6 1. 6

T5151 9.3 16.6 5.6 5.6 5.6V 9.5 5.6 5.6 16.6 v1.6

as weil as Matrix.

inch. The resultant compact is then sintered to cause coalescence oralloying of the alloyable materials which form into an integrated massor matrix having a degree of porosity, .the pores serving as pockets forcontaining the non-alloyable materials. Suitable sintering temper-aturesrange between 1100" F. and 1900 F., and sintering times may rangebetween 20 minutes to 11/2 hours.

By way of a specific example, and following the above procedure, acomposition is formed in which a metal matrix constitutes ofthecomposition and consists of copper, zinc, land tin in the relativeproportions of 84:10:6. Combined with the matrix material is 8% of iron,24% of calcined kyanite, 4% of silica, 8% of graphite and about 5% ofmolybdenum. The foregoing materials, in lnely-divided Y form, arecompacted and sintered 'in `a ferrous retaining member. The composition,when heated from kinetic energy absorbed during braking, was found .toprevent objectionable Vchatter of the brake.. '5

'This data is based on the use of calcined kyanite as the principalceramic material. However, composition l7 lhas been rtested withsuccessful results using mullite bearing materials of both typespreviously described. The braking compositions of this invention may bedescribed as having `a metallic matrix, a prime vfrictionpr-oducingceramic material and an antioxidant. Stated thus broadly, these are theessential constituents of the novel compositions. The brakingcompositions are provided in a friction-producing article having anengaging surface which is self-compensating Ifor effects of oxidationinduced by high temperatures encountered during the braking cycle. Byvirtue of the inclusion of an antioxidant, the article is adapted todeliver torque with sufciently dampened rate of change in eifectivenessapproaching the end `of a high speed `objectionable brake chatter. Y

Theceramic materialemployed may be silica, uncaleined (raw) kyani te,alumina or other abrasiverefrac-Y stop to prevent tory material.Specific examples of compositionshaving such materials are thefollowing:

Friction Material:

TOtal 4. 7

As was expected based on our knowledge of the function v of theantioxidant, in tests of the three foregoing compositions (Nos. 21-23),the molybdenum produced an yimproved brake performance of --themetal-ceramic compositions Aby preventing the torque build-up during astop which is believed productive of'nobjectionable brake chatter. Y YThe background ceramic plays noessential role in`the functioning of theantioxidant. As pointedoutpreviously in the description ofthefunctionofthe molybdenum, the kmolybdenum accomplishes its result bycontrolling the Voxide composition of the glaze and-it does this byvirtue of its aiiinity'for oxygen. Y l vThe lfact that the molybdenumdoes operate to advantage irrespective of the ceramic further confirmsour-belief that the molybdenumaccomplishes its resultsfor the 4reasonsstated. The compounder has a *Wide latitude in the selection of aprimary friction producing ceramic and can use whatever refractoryabrasive oxide isv suggested by the particular requirements. VWe havefound, however, Vthat the mullite has inherently desirable propertieswhich make it a preferred selection in compounding many aircraftbrakeliners. This superiority of mullite is not attributable lto theantioxidant, however; nordoesthemullite influence the molybdenum in itslassigned function. Referring to Figure 3, there is illustrated thetorque recording of a typical brake ,composition of k'Iable I,

formulation 13. This recording is a torque y.v speed braking historyobtained with astandard 84 inch inertia brake dynamometer whichsimulates actual aircraft Abraking conditions. j Figure 2 is the torqueV. speed curve of a conventional ceramic-metallic lining with the sameingredients as formulation 13, but without an antioxidant such asmolybdenum and a slight changeqin the s ilica content. The compositionof the lining used in obtaining the curve in Figure 2 is as follows:

Matrix:

' Copper 45.0 Zinc 10.0 Iron'1 5.0

' Totall 60.0

1This material in addition ,to being a non-alloying ingredient of thematrix also serves as a. secondary friction producing agent and is,therefore classitlable Vunder Friction Material as welles Matrix. n

Frictionmaterialx.

vso

Antioxidant:

' Molybdenum 0.0 Total 0.0

In interpreting these curves, special emphasis will be placed onthe lastthree second interval of the brake cycle, for it is in this `period thattorque build-up can produce ythe objectionable chatter of Ythe brake. Ithas been found by actual experience that 4brake chattering is not onlyrelated to the amount of torque build-.up but also the rate of whichthis build-up'occurs. 'Ihe change in effectiveness of the compositionwhich causes the torque buildup during the braking cycle is relativelyunimportant at -the higher speeds of the aircraft; it becomes importantin relation to chatter only during the iinal seconds of the stop, whenspeeds of the craft approach ranges of 5,0 m.p.h.k and below.

Torque build-up and-resulting chatter can be predicted fromv thetorque-velocity curve which records the braking history both as toabsolute amount of torque and also the rate of build-up of the torque;At speeds of the aircraft during the last three seconds or so of thebraking action, a torque build-up both as to amount and rate oflincrease is especially critical, since at the lower speeds theefrects oftorque build-up are more noticeable and are reilected as ya pronouncedharmonious vibration of the undercarriage. With these principles inview, refer now to the torque curve in Figure 2, noting especially thelast three second interval of the braking cycle. It will be seen thatthere is a sharp increase in torque delivered by the ,brake at theend ofthe stop. It is this condition of torque increase which produces brakechatter Vwith the objectionable results hereinbefore mentioned.

Turning now to Figure 3, the effect of adding molybdenum to the liningwill be demonstrated. Analyzing this torque-speed curve, it is seen`that in the interval near the end of the stop, the torque decreasesslightly. In other words, the brake lining effectiveness at lowvelocities does not have an objectionable torque build-up. Such atorquespeed curve demonstrates .the braking pattern of a chatterfreebrake, one whose characteristics are substantially free of theobjectionable results which accompany torque buildup. Further than thisthe torque build-up has been eliminated `though the ceramic content isabout 27% of the brake lining. ,This result has been accomplished by useof molybdenum antioxidant, according to formulation 13 listed inTableI.. Y Y v Figure 1 illustrates a button testing machine 16 used topreliminarily indicate the braking pattern or friction characteristicsof a composition. specimen to determine .whether itrwi'll cause brakechat-ter. The operation of thisfmachine is asfollows:

Arm 18 is `driven through` shaft 20 by a variable speed motor 21 andhas. mounted at oppositeends 24 and 26 brake specimens 14 which are`caused to rotate in a circular path aboutshaft 20. The arm 18 is urgedupwardly by a spring 28 compressed between collartl'and arm 1S therebyYyieldably suspending arm 18 and brake specimens force of engagement ofbrake' specimens 14 with the heated circular plate 40 corresponds to theactuating force of a brake application and may be varied by changing thelever arm of weight 36. y

. Indi'catingV devices 42 are connected to appropriate parts of theVbutton testing machine `116 to Ydetermine Such values VVas brakingtorque and temperature. During the period l of engagement of the brakespecimens 14 with plate 4i) recordings are made ofthe braking torque atappropriate periods. The motor 21 is stopped and the vchattercharacteristics of the brake specimens Iare determined from the torqueincrease during deceleration of the rotating members from a fixedvelocity to zero velocity. Lining specimens which prove satisfactory inthis test are then tested on 'a standard 84 inch inertia -brakedynamometer, which simulates the actual braking conditions encounteredin eld service.

The surface of the brake lining in the present invention is designed toundergo a selected Aamount of melting and/or other physical changes atappropriate stages of the braking cycle. This controlled melting and/orother physical changes permit timely additional lubrication which inturn prevents chatter of the brake. In view of extensiveexperimentation, a theory linking physical change ofthe surface of thelining to the prevention of torque build-up appears to provide aplausible reason for the elimination of chatten Physical change of thesurface of the vlining in the form of mel-ting is substantiated inFigure 4. This is a photomicrograph, 174 magnification, illustrating across section of the composition i2 after a series of high-speed brakingstops. It will be seen that glaze 44 has melted under the iniiuence ofheat into voids 45 which are formed in the surface of the lining. Otheridentifiable materials in the lining are kyanite grains 48.

In formulating compositions including the various ingredients-matrix,lubricant, antioxidant, and prime friction-producing ceramic-it has beennecessary to strike some balance between qualitiestof the brake whichare opposed to one another. But, on the whole, the braking compositionsset out in Table il will produce linings which provide to an optimumextent effectiveness, long wear, and chatter-free braking. t

Although only selected formulations have been described, it will beobvious to those skilled in the art that Vvarious changes in the liningingredients may be made to suit requirements.

We claim:

1. For use in high kinetic energy braking, a friction com-positionconsisting essentially of the following approximate percentages byweight: 557% of matrix material consisting of copper, zinc and tin intheapproximate relative proportions ofv 84:10:6, about 8% .ofmatrix-insoluble iron for producing increased brake effectiveness, about24% of a primary friction-producing ceramic material consistingprincipally of calcined kyanite embedded in said matrix in the form of aseries of dispersed granule structures and including about 4% of silicafor yadding to the effectiveness of the brake, about 8% of acarbonaceous lubricating material in the form of graphite, and about ofmolybdenum interspersed in said composition, said brake compositionbeing compacted and sintered in a ferrous retaining member to provide afriction article adapted for engagement with a relatively rotatablemember, said composition being thereby heated from kinetic energyabsorbed during braking in such a manner that changes in the frictionproperties of s-aid composition will-tend to prevent chatter of thebrake.

2. A friction producing device wherein two surfaces are rubbed togetherto produce surface temperatures above approximately a red heat, andwherein one of the rubbing surfaces includes `a readily oxidizable formof iron: one of Asaid Asurfaces being formed by a sintered powderedmaterial consisting principally of a powdered metal ymatrix whichremains solid at operating conditions and having a ceramic frictionproducing material dis.- trbuted uniformly therethrough, said lastmentioned one ofsaid surfaces further including from about v1% tosisting of molybdenum and tungsten for the purpose of inhibiting theformation of the higher oxides of iron upon the surfaces.

'3. A friction producing device wherein two surfaces are rubbed togetherto produce surface temperatures above approximately a red heat, andwherein one of the rubbing surfaces includes a readily oxidizableiformof iron: one of said surfaces being formed by a sintered powderedmaterial consisting principally of a powdered metal matrix which remainssolid at operating conditions and having Afrom approximately 15% toapproximately 43% by weight of a ceramic friction producing materialdistributed uniformlyY therethrough, said last mentioned one of saidsurfaces further including from about 1% to about 121/2% by weight ofY ametal from the group consisting of molybdenum and tungsten for thepurpose of inhibiting the formation of the higher oxides of iron uponthe surfaces.

4. A friction producing device wherein two surfaces are rubbed togetherto produce surface temperatures above approximately a red heat, andwherein one of the rubbing surfaces includes a readily oxidizable formof iron: one of said surfaces being formed by a sintered powderedmaterial consisting principally of a powdered metal matrix which remainssolid at operating conditions and having a ceramic friction producingmaterial distributed uniformly therethrough, said last mentioned one ofsaid surfaces further including from lapproximately 1% to approximately121/z% by weight of atoms of molybdenum which oxidize at operatingconditions for the purpose of inhibiting the Aformation .of the higheroxides of iron upon the surfaces.

5. A friction producing device wherein two surfaces are lrubbed togetherto produce surface temperatures above approximately a red heat, andwherein one of the rubbing surfaces includes a readily oxidizable -forrnof iron: `One of said surfaces being formed by a sintered powderedmaterial consisting essentially of from approximately 35% toapproximately 65% by weight of a powdered metal matrix which vis solidat operating conditions, from approximately 15% to approximately 43% byweight of a ceramic friction producing material distributed uniformlytherethrough, VIand, from about 1% to about l21/2% by weightof a metalfrom the group consisting .of molybdenum and tungsten for the purpose ofinhibiting the formation of the higher oxides of iron upon the surfaces.t

6. A friction producing device wherein two surfaces are rubbed togetherto produce surface `temperatures above` approximately ya red heat, andwherein one of the rubbing surfacesgincludes Va readily -oxidizable.form of iron: one of said surfaces being [formedrby a sintered powderedmaterial consisting essentially of from approximately 35% tovapproximately 65% by weight of a powdered metal matrixwhich is solid atoperating conditions, from approximately l15% to approximately 43% byweight of a ceramic friction producing material distributed uniformlytherethrough, and .approximately 5% by weight of molybdenum for thepurposes of inhibiting the formation of the higher oxides of iron uponthe surfaces. 1 A

7. Afriction producing device wherein two-surfaces are rubbed togetherto produce Vsurface -temperaturesabove approximately la red hea-t, Y-andwherein .oneof the nub- Ahing surfaces vincludes a readily oxidizableform of iron: one .of said surfaces being formed by a sintered powderedmaterial consisting essentially of a powdered met-al matrix having atleast one ceramic friction producing material fromthe group consistingof calcined kyanite,Y

raw kyanite, alumina and silica," a carbonaceops lubricant, i' and Afrom`Iap'pr'oximately 1% to approximately 121/2 by Weight of Va materialfrom the group consisting enf-.atoms of molybdenum and tungsten which.oxidize lat v 8. A friction producing device wherein two surfaces arerubbed together to produce surface temperatures above approximately ared heat, and wherein one of the rubbing surfaces includes a readilyoxidizable form of iron: one of said surfaces being formed by auniformly mixed and sintered powdered material consisting essentially ofa powdered metal matrix having at least one ceramic friction producingmaterial from the group consisting of calcined kyanite, raw kyanite,alumina, and silica, a carbonaceous lubricant, land from approximately1% to approximately 121/2% by weight of powdered molybdenum having aparticle size of approximately 200 mesh, and whereby chatter Iand torquebuild-up are inhibited.

9. A method of friction control for high energy absorbing aircraftbrakes and the like of the type having two structures forming surfacesthat are rubbed together in an oxidizing atmosphere, one of ysaidstructures being formed of a sintered powdered material consistingprincipally of a powdered metal matrix having a ceramic frictionproducing material distributed uniformly therethrough, and at least oneof said structures also including a readily oxidizable form of iron,said method comprising: providing one of said structures with fromapproximately 1% to approximately 121/z% by weight of a material of theclass consisting of atoms of molybdenum and tungsten which oxidize atthe rubbing temperature, and forcing said structures together withsufficient rubhing force to generate local surface friction temperaturesin excess of approximately 2500 F. to form a molten oxide glaze in whichthe formation of the higher oxides of iron are substantially preventedby the preferential oxidation of the molybdenum and/ or tungsten atoms.

10. Amethod of friction control for high energy absorbing aircraftbrakes and the like of the type having two structures forming surfacesthat are rubbed together in an oxidizing atmosphere, one of saidstructures being formed of a sintered powdered material consistingprincipally of a powdered metal matrix having a ceramic fricf tionproducing material distributed uniformly therethrough, and at least oneof said structures also including a readily oxidizable form of iron,said method compn'sing: providing one of said structures with fromapproximately.1% to approximately 121/2% by weight of atoms ofmolybdenum which oxidize at the rubbing temperatures, and forcing saidstructures together with sufcient rubbing force to generate localsurface friction temperatures in excess of approximately 2500 F. to forma molten oxide glaze in which the formation of the higher oxides of ironare substantially prevented by the preferential oxidation of themolybdenum atoms.

11. A method of friction control for high energy absorbing aircraftbrakes and the like of the type having two structures forming surfacesthat are rubbed together in an oxidizing atmosphere, one of saidstructures being formed of a sintered-powdered material consistingprincipally of a powdered metal matrix having a ceramic frictionproducing material distributed uniformly therethrough, and at least oneof said structures also including a readily oxidizable form of iron,said method comprising: providing one of said structures withfromapproximately 1% to approximately l21/2% by weight of molybdenum andforcing said structures together with suiicient rub bing force to form amolten iron oxide glaze in which the formation of the higher oxides ofiron are substantially prevented by the preferential oxidation of themolybdenum atoms.

References Cited in the file of this patent UNITED STATES PATENTS2,072,070 Fisher Feb. 23, 1937 2,307,512 Kelly Ian. 5, 1943 2,389,061Kuzmick Nov. 13, 1945 2,408,430 Lowey et al, Oct. 1, 1946 2,470,269Schaefer May'17, 1949 OTHER REFERENCES Norton: Refractories, 1931, lstedition, published by McGraw-Hill Book Co., New York, page 191.

UNITED STATES PATENT OFFICE CERTIFICATION 0F CGRRECTION Patent No.241948,955 August l, 1960 Alfred W. Allen et al.

lt is hereby oertfedthat error appears in the above numbered patentrequiring correction and that the said Letters Patent should read asCorrected below.

Column 4, line 35, for "producting"` read producing line 65, for "SiO"read S102 Signed and sealed this 4th day of July 1961.

(SEAL) Attest:

ERNEST W. SWIDER DAVID L. LADD Attesting Officer Commissioner of Patents

